Potential demethylation rate determinations in relation to concentrations of MeHg, Hg and pore water speciation of MeHg in contaminated sediments

Specific, potential demethylation rate constants (k(d), day(-1)) were determined in fresh and brackish water sediments from seven different sites in Sweden originally contaminated with either He(I) or phenyl-Hg. Variations in kd among and within sites were related to ambient concentrations of Hg (1-1143 nmol g(-1)) and MeHg (4.4-575pmol g(-1)), and to pore water speciation of MeHg. Chemical speciation modeling revealed that MeHgSH(aq), MeHgS-(aq) and MeHg-thiol complexes[MeHgSR(aq)] associated to dissolved organic matter were the dominant MeHg species in the sediment pore water at all sites. Potential rates of MeHg demethylation were determined as the decomposition of isotopically enriched (MeHgCl)-Hg-204 during 48 h of incubation in darkness under N-2(g) at 23 degrees C. There was a significant (p < 0.001) positive relationship between ambient MeHg concentrations in sediments and kd across all sites, but no significant relationship between ambient Hg and kd. At the three sites with the highest ambient Hg concentrations in sediments (average +/- SD, 185 +/- 249 nmol g(-1)), kd was not significantly correlated with pore water MeHg speciation. At sites with lower concentrations of ambient Hg in sediments (average SD,11 8.4 nmol g(-1)), there was a significant (p = 0.02) positive relationship between calculated concentrations of MeHgSH(aq), MeHgS-(aq), or the sum of these two species, and kd. If it is assumed that an oxidative demethylation process dominated at sites with lower concentrations of ambient Hg in sediments, the results suggest that it may be dependent on a passive uptake of inorganic MeHgSH molecules. It was shown that additions of different amounts of MeHg and Hg tracers, in relation to the ambient concentrations of MeHg and Hg, could result in dramatically different kd values within and between sites. At one brackish water site, both absolute demethylation rates and k(d)s were significantly, inversely related to ambient concentrations of MeHg (and Hg). In contrast, at another brackish water site with generally less kds, samples with low ambient MeHg experienced toxic effects and demethylation was not detected. This implies that added (and possibly ambient) MeHg/Hg, depending on the environmental conditions, may have either stimulating or inhibitory effects on demethylation processes. (C) 2008 Elsevier B.V. All rights reserved.